Alton Sterling, a 37-year-old black man, was shot and killed by officers of the Baton Rouge Police Department last summer. The shooting was captured on video and immediately ignited protests in the community.Published OnJuly 6, 2016CreditImage by William Widmer for The New York Times

BATON ROUGE, La. — The Justice Department opened a civil rights investigation on Wednesday into the fatal shooting of a black man by the Baton Rouge, La., police after a searing video of the encounter, aired repeatedly on television and social media, reignited contentious issues surrounding police killings of African-Americans.

Officials from Gov. John Bel Edwards to the local police and elected officials vowed a complete and transparent investigation and appealed to the city — after a numbing series of high-profile, racially charged incidents elsewhere — to remain calm.

“I have full confidence that this matter will be investigated thoroughly, impartially and professionally,” Mr. Edwards said in announcing the federal takeover of the case. “I have very serious concerns. The video is disturbing, to say the least.”

Urging patience while the investigation takes place, the governor said: “I know that that may be tough for some, but it’s essential that we do that. I know that there are protests going on, but it’s urgent that they remain peaceful.”

Two white officers were arresting Alton B. Sterling, 37, early Tuesday after responding to a call about an armed man. The officers had Mr. Sterling pinned to the ground when at least one of them shot him.

The video of the shooting propelled the case to national attention, like a string of recorded police shootings before it. The shooting has prompted protests here in the Louisiana capital, including a vigil with prayers and gospel music that drew hundreds of people Wednesday night to the storefront where it happened.

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A YouTube image of Alton Sterling.CreditYouTube

C. Denise Marcelle, a state representative who recently announced that she would run for mayor, made impassioned pleas that the crowd remain calm.

Sandra Sterling, an aunt who said she had raised Mr. Sterling, also called for peace. “I’m mad,” she said, but added, “I’m not angry enough to hurt nobody.”

LaMont O. Cole, a city councilman, had some of the harshest words for the two police officers. “Those two officers who perpetrated this brutal attack, and then murdered this young man, are cowards,” he said.

The decision to have the Justice Department’s Civil Rights Division, the F.B.I. and the United States attorney’s office in Baton Rouge conduct the investigation was welcomed by a lawyer for Mr. Sterling’s family.

“We’re confident that it won’t be swept under the rug,” said the lawyer, Edmond Jordan, who is also a state representative. “I think people are confident that justice will be pursued.”

Officials identified the officers as Blane Salamoni, who has been with the force for four years, and Howie Lake II, with three years’ experience. Both were placed on administrative leave.

A call to a phone number for Mr. Salamoni was answered by a man who said he was not the officer, but who would not identify himself. “When all the facts come out, they did what they had to do,” the man said, and then hung up.

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Protesters along North Foster Drive on Wednesday in front of the Triple S Food Mart.CreditWilliam Widmer for The New York Times

Mr. Salamoni is the son of Noel Salamoni, a captain in the department who is in charge of special operations.

Local and state officials endorsed the federal takeover of the case. “We feel it is in the best interest of the Baton Rouge Police Department, the city of Baton Rouge and this community for this to happen,” the police chief, Carl Dabadie Jr., said.

In other cities with high-profile deaths of people in police custody, when local law enforcement agencies have kept control of the investigations and prosecution, they have often drawn intense criticism for their handling of the cases.

There are multiple videos that may show the conflict with Mr. Sterling, in addition to the one recorded by a bystander that has been made public, said Lt. Jonny Dunnam, a police spokesman, at a news conference. Mr. Jordan, the family lawyer, called on the police to release the videos, but Lieutenant Dunnam said that for now, the department was providing them only to the federal authorities.

“We have in-car camera video footage, we have body camera video footage and there is video at the store,” Lieutenant Dunnam said. Of the recordings from the body cameras the officers wore, he said: “That footage may not be as good as we hoped for. During the altercation those body cameras came dislodged.”

At an earlier news conference on Wednesday, family members, elected officials and civic leaders demanded to know why Mr. Sterling had been killed. Some of them, including the local N.A.A.C.P. president, Mike McClanahan, called on Chief Dabadie to resign.

Relatives and friends of Alton Sterling, 37, who was shot and killed outside a convenience store in Baton Rouge, La., on Tuesday, spoke about his death.Published OnJuly 6, 2016CreditImage by Bill Feig/The Advocate, via Associated Press

“The individuals involved in his murder took away a man with children who depended upon their daddy on a daily basis,” Ms. McMillon said, adding, “As a mother I have now been forced to raise a son who is going to remember what happened to his father.”

On Tuesday, a person called the police to report that a black man in a red shirt selling music CDs outside the Triple S Food Mart had threatened him with a gun, the Police Department said. Two officers confronted Mr. Sterling about 12:35 a.m.

Mr. Sterling had a long criminal history, including convictions for battery and illegal possession of a gun, but it is not clear whether the officers knew any of that as they tried to arrest him.

The graphic cellphone video shot by a bystander, which was released later in the day, shows an officer pushing Mr. Sterling onto the hood of the car and then tackling him to the ground. He is held to the pavement by two officers, and one appears to hold a gun above Mr. Sterling’s chest.

At one point someone on the video can be heard saying, “He’s got a gun! Gun!” and one officer can be seen pulling his weapon. After some shouting, what sound like gunshots can be heard and the camera shifts away, and then there are more apparent gunshots.

A second video of the shooting, filmed by the owner of the store and first posted by the local newspaper, The Advocate, on Wednesday afternoon, showed the shooting from a different angle. It also shows one of the officers taking something out of Mr. Sterling’s pocket after he was shot and was lying on the ground.

Witnesses have said they saw a handgun on the ground next to him. Mr. Jordan, the lawyer, said Mr. Sterling’s relatives were not aware of him owning a gun.

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A portrait of Mr. Sterling, shot to death early Tuesday, was painted Wednesday on the Triple S Food Mart near where he was killed.CreditWilliam Widmer for The New York Times

Arthur Reed, the founder of Stop the Killing, the group that released the cellphone video, said he saw a gun only after Mr. Sterling had been fatally shot. The group, a mentoring program for youths, had heard reports on a police scanner about an arrest at the store, and showed up to gather video for potential use in a documentary about urban violence.

Mr. Reed said the group decided to release its video after he heard that the police had accused Mr. Sterling of reaching for a gun.

“He never reached in the video,” Mr. Reed said. “He never did anything.”

William Clark, the coroner of East Baton Rouge Parish, said that Mr. Sterling had died at the scene from gunshot wounds to the chest and back. Lieutenant Dunnam declined to say whether both officers fired their guns, or if either of them used an electric stun device on Mr. Sterling.

Mr. Sterling’s name began trending on Twitter Tuesday night. In a statement on the killing, Hillary Clinton said, ”Something is profoundly wrong when so many Americans have reason to believe that our country doesn’t consider them as precious as others because of the color of their skin.”

By Wednesday evening, the parking lot of the Triple S was jammed with protesters and TV cameras. The protesters, young and old and nearly all African-American, waved signs declaring that black lives matter.

Anthony Anderson, 62, a tour bus driver, and his cousin, David Jones, 60, who is self-employed, said they had had enough.

“I just think it looked like there could have been another way to handle that situation,” Mr. Anderson said of the video. He said that it seemed to him that the police here had long been harassing black people.

The videos made just as little sense to Leroy Tackno, 60, the manager of the Living Waters Outreach Ministry transitional housing center where Mr. Sterling kept a small bedroom for $90 a week. He said that Mr. Sterling had never been any trouble.

“I’m just trying to figure out what did he do,” Mr. Tackno said. “All he did was sell CDs.”

Richard Fausset reported from Baton Rouge, Richard Perez-Pena from New York and Campbell Robertson from New Orleans. Reporting was contributed by Mike McPhate and Jonah Engel Bromwich from New York, Allen Johnson from Baton Rouge and Timothy Williams from Washington. Alain Delaquérière contributed research.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: U.S. Examines Police Killing in Louisiana. Order Reprints | Today’s Paper | Subscribe